1. Field of the Invention
This invention pertains generally to topographic mapping and more specifically to the measurement of topography over a wide area utilizing a polarimetric synthetic aperture radar (SAR) installed in an aircraft or satellite.
2. Description of the Related Art
Wide-area measurements of topography currently utilize either stereo-photography or interferometric synthetic aperture radar (IFSAR). However, stereo-photography is limited by the weather, atmosphere (absorption, refraction, turbulence, etc.) and darkness.
Microwave interferometric SAR has shown promise as a wide-area technique for the measurement of topography but has its own set of limitations. Radar mapping utilizing interferometric principles was first applied in the late 1960's and early 1970's to earth-based observations of Venus and the moon. The simultaneous measurement of range, azimuth angle, and elevation angle provides the absolute three-dimensional location of each image point, thus radar images can be accurately transformed from slant range to ground range format. This enables the correction of radar artifacts such as foreshortening and simplifies the coregistration of diverse images. The combination of one interferometric channel with simultaneously operated channels (multifrequency, multipolarization) will permit automatic generation of multichannel three-dimensional geolocated radar images. The interferometric technique utilizes the property that a small time delay difference is associated with a phase shift. Phase measurements made by interferometric SAR provide the opportunity for accurate elevation measurements, however, a different set of measurement problems are created. The major phase measurement problems involve (1) exact knowledge of the scattering path geometry, (2) platform motion compensation, and (3) 2.pi. phase ambiguities caused by abrupt elevation changes which require "unwrapping" algorithms and constitute recovering the multiples of 2.pi. phase differences that "disappear" in an interferogram. See, Madsen et al., Analysis and Evaluation of the NASA/JPL TOPSAR Across-Track Interferometric SAR System, IEEE Trans. Geosc. and Remote Sensing, Vol. 33, No. 4, pp. 383-391, 1995; E. Rodriguez et al., Theory and Design of Interferometric Synthetic Aperture Radars, IEEE Proc.-F, Vol. 139, No. 2, pp. 147-159, 1992; Li et al., Studies of Multibaseline Spaceborne Interferometric Synthetic Aperture Radars, IEEE Trans. Geosc. and Remote Sensing, Vol. 28, No. 1, pp. 88-97, 1990; and U.S. Pat. No. 4,975,704, Gabriel et al.
Forest areas present additional problems for interferometric SAR since practical antenna baseline separations for aircraft and single satellite applications generally imply the use of microwave frequencies too high to penetrate forest canopy to the ground. Polarimetric SAR at the P-band and lower frequencies provide a method for the measurement of topography in forested areas and may be used as a measurement tool which can aid in solving the phase ambiguity problems of interferometric SAR. Polarimetric SAR slope and elevation measurements can provide bounds on the IFSAR phase path. Finally, the relative simplicity of fully-polarimetric real, and synthetic aperture, radar design has resulted in a rapid expansion in the number of such systems, application and datasets. Many of these datasets involve geophysical parameter measurements which may utilize the techniques taught by this invention to improve the estimate of the parameter.